1. Field of the Invention
The present invention relates to the stabilization of an image formed by an image forming apparatus such as a copying machine, a printer or the like.
2. Description of the Prior Art
In general, the electrostatic characteristic of a photoconductor used in electrophotographic process varies with environment and wear of a photoconductor on which a toner image is formed. Further, it also varies among photoconductors. Especially for a color image, such a change of electrostatic characteristic of photoconductor affects largely the reproduction of colors, the balance of colors and the reproduction at low densities.
Then, in order to stabilize the reproduction of an image, automatic image density control (AIDC) is performed before reproducing an image to determine image-forming conditions so as keep the image density stable. In the automatic image density control, a standard toner image for the automatic image density control is first formed at a half-tone level on the surface of the photoconductor, and the toner density of the standard toner image is measured by detecting the quantity of reflection light with a sensor arranged near the photoconductor. Then, image-forming conditions on reproducing an image such as the maximum quantity of exposure light, the surface potential, the gradation characteristic and the like are controlled according to the detected toner density. Further, in order to improve the precision of the above-mentioned control of the image-forming conditions, for example, the potential of an electrostatic latent image formed before the automatic image density control is measured, and the image-forming conditions for the standard toner image formed in the automatic image density control is adjusted for example on the surface potential, the quantity of light or the like.
In a method for adjusting the image-forming conditions on the automatic image density control, the surface potential of a portion to be exposed is corrected before the exposure, and the surface potential of the exposed portion is detected after exposing the portion at the standard quantity of light. Then, the maximum quantity of light is determined according to the relation between the detected value and the quantity of light. For example, in a copying machine, after a photoconductor is sensitized with a standard grid potential of a sensitizing charger, the surface potential of the photoconductor is detected and a correction amount of the surface potential is determined according to the detected value. Next, the surface potential is detected after the photoconductor is exposed with a standard quantity of light, and a corrected maximum quantity of light is determined according to a predetermined relation of the detected surface potential value with the quantity of light. Then, a standard toner image is formed in automatic image density control by using the grid potential and the maximum quantity of light both corrected as explained above, and the image-forming conditions such as the grid potential of the sensitizing charger, the development bias potential of a development unit and the gradation characteristic are determined according to the detected toner density of the standard toner image. Then, a copy action is carried out in the image-forming conditions determined by the automatic image density control. Such correction of image-forming conditions of a latent image in the automatic image density control improves the precision of the automatic image density control, so that the change in the sensitivity of the photoconductor due to environment or wear can be corrected to a certain degree.
In this method, in order to shorten a time needed to start up in a copying machine, the potential of a standard pattern prepared only at a quantity of light is measured before the automatic image density control. This method has an advantage that a time needed for detection is short and the start up time before the automatic image density control does not become long.
However, this method has a disadvantage that the correction may not become appropriate in some conditions. For example, if the grid potential of the sensitizing charger change from the value just after the initial correction due to environment such as high humidity, the surface potential which will be expected to be realized according to the automatic image density control cannot be obtained.
Further, in a multi-copy wherein copies are produced continuously, the potential of the photoconductor (especially after irradiated with light) may vary with the repetition of sensitizing and erasing of the photoconductor. In such a case, even if the surface potential is detected at a portion between latent images and the image-forming conditions are corrected according to the detected value, the correction precision is liable to become worse because the potential at the image forming is different from that measured for the correction.
In order to perform the automatic image density control precisely, it is desirable to keep the surface potential of a photoconductor before and after the exposure constant under any conditions for forming the standard toner image, so as to make the development voltage constant.